1. Field of the Invention
This invention relates generally to systems and techniques for displaying electronically images generated from acoustic data. More particularly, the invention is related to means and methods for displaying medical ultrasound images that contain color flow information.
2. Description of the Background Art
Diagnostic ultrasound techniques currently utilize ultrasound to obtain information about the nature and structure of tissues and organs of the human body. This technique has become widely accepted in medicine, partly because it allows a physician to obtain pictorial cross sections of various parts of the body in a noninvasive way, and in some medical specialties, such as obstetrics, ultrasound techniques have virtually supplanted all other imaging methods.
Medical diagnostic ultrasound systems generally yield information concerning structures within the body of a patient by transmitting sound waves of very high frequency, typically on the order of several megahertz, from a transducer into the patient's body and analyzing the echoes reflected from these structures that are detected by the transducer. The transducer operates both as the transmitter and the receiver of the acoustic waves. The information obtained from these echoes can be displayed and studied in several ways, referred to as "modes". The display of information is often on a cathode ray tube (CRT) screen in real time, during the actual examination of the patient.
For example, the A-mode ("Amplitude") display is a plot of the amplitude of the reflected signals as a function of time. The delay time between a transmitted sound wave pulse and a received echo pulse determines the distance from the transducer to the reflecting structure. Furthermore, the acoustic pulses may be focused into a beam along a given direction. This focusing is preferably achieved by utilizing a phased array of transducers, rather than a single transducer. This method allows the user to steer the acoustic beam and adjust the focus dynamically in real time. Therefore, the direction of propagation of the stimulus and echo signals is also determined, and the A-mode display provides information about the location of the reflecting structures along a beam line.
A clearer picture of the geometry of the reflecting structures is provided by the B-scan ("Brightness") display, which represents the amplitude of the reflected signals from a given direction as the brightness of the line along a CRT trace. The distance along the trace is proportional to the distance from the transducer to the reflecting location. By scanning a focused beam of transmitted signals across a sector under examination and simultaneously sweeping the CRT trace in a direction perpendicular to the trace axis ("B-mode"), the resulting display shows reflecting interfaces along a collection of beam lines in a plane section through the body. Thus, the B-mode display comprises a two-dimensional image of a cross section of the structure being studied. With presently available technology these images can be generated and displayed in a real-time mode during the examination of a patient. Thus the B-mode technique is a very useful tool in a clinical environment.
The foregoing B-mode method produces images that may include a direct view of some tissue motion, such as heart wall movement. However the resolution limits of the ultrasound technique imply that smaller structures cannot be studied by this method. This limitation can be significant in some applications, such as cardiac imaging, for example, where important structural features may be too small to be imaged by the B-mode ultrasound method. In such applications it is desirable to obtain information regarding blood flow through the structure, because variations in the velocity of blood flow can reflect diagnostically important anatomical characteristics and abnormalities. For example, vascular obstruction may manifest itself through abnormally high blood flow velocity.
The color flow imaging method is an improvement on the B-mode technique that is designed to include information about blood flow in the ultrasound scan images. This method is described in U.S. Pat. No. 4,800,891, issued Jan. 31, 1989 (Kim), assigned to the assignee of the present invention. The technique involves the measurement of the Doppler shift of acoustic signals reflected from moving targets, such as flowing blood. The Doppler shift in the reflected sound wave frequency is proportional to the projection of the blood flow velocity along the beam axis between the transducer array and the sample point in the blood flow where the reflection originates. The reflected signal therefore carries information about this projected flow velocity. In the color flow imaging method, the projected velocity component is represented as a certain color representing the direction of the flow, and the color intensity varies directly with the magnitude of the blood velocity.
The color flow technique is implemented together with the gray-scale B-mode technique described above by using the pulsed Doppler method. The acoustic stimuli are a train of pulses focused along a beam ray into the body, and this ray direction is swept through the sector under examination. The position of the axis and the arrival time of the reflected pulses at the transducer determine the location of each reflecting sample. In addition, the frequency shift of each reflected pulse determines the velocity projection along the beam ray of the reflecting sample. For each such reflecting sample in the flow pattern, a spot having a color representative of the blood flow velocity is superimposed upon the above-described B-mode image. The composite image thus comprises the gray-scale B-mode image showing the geometry of various structures, together with colored areas within these structures indicating blood flow.
These real-time image-forming techniques generally require a device, termed a "scan converter", for converting the information contained in the reflected acoustic signals into a form suitable for display on a CRT television-type screen. The scan converter is a memory into which information is written in a format corresponding to the acoustic signals received by the transducer, and from which this information is read in a format suitable for CRT display, such as a standard television raster pattern. While the early scan converters were analog devices, digital scan converters (DSC) have been found to be advantageous for medical ultrasound applications. DSC technology has been reviewed in an article by J. Ophir and N. F. Maklad, "Digital Scan Converters in Diagnostic Ultrasound Imaging", published in Proceedings Of the IEEE, Vol. 67, No. 4 (April, 1979).
The DSC transforms the coordinates indicating the location of the acoustic reflection points into corresponding positions on the CRT display screen. Acoustic data are typically gathered by scanning the beam rays extending from the transducer array through a sequence of angles traversing the sector of the body under examination. In a given ray the reflected signals are labeled by the distance along the ray from the transducer to the point of reflection. The locations of the reflecting samples are thus specified in polar coordinates in the plane of the sector. However in a standard television display the location of image pixels on the screen is specified in terms of the usual Cartesian x-y coordinates. In the standard display system the DSC carries out a mapping of the acoustic data from polar to Cartesian coordinates.
In addition, the DSC serves as a buffer to compensate for the mismatch between the rate at which data are collected by the acoustic sampling system and the rate at which they can be exhibited by the video display. This function is described in U.S. Pat. No. 4,449,199, issued May 15, 1984 (Daigle).
The coordinate transformation by the DSC typically produces a number of undesirable artifacts in the resulting image, since only a finite number of pixels and polar data sample locations can be processed, and the display at most pixel locations must be interpolated from neighboring polar data sample locations. The interpolation is normally limited to bilinear interpolation between nearest neighbor sampling points. These artifacts can include a blocky appearance of structures, oversampling of some data, and the well-known Moire artifact. An article by M. H. Lee, J. H. Kim and S. B. Park, "Analysis of a Scan Conversion Algorithm for a Real-Time Sector Scanner", published in IEEE Transactions on Medical Imaging, Vol. MI-5, No. 2 (June 1986) reviews many of these problems in the gray-scale B-mode context, along with some of the solutions that have been proposed.
The previously known interpolation techniques for scan converters present further problems when they are used in color flow imaging systems. The images require interpolation of data representing flow velocity as a function of position, as well as the locations of solid structures. According to the conventionally preferred methods as described in Kim, supra, the actual flow velocity data are contained in estimates of the correlation value between pairs of acoustic pulses reflected from the sample region. The correlation value is preferably the first lag autocorrelation, although in principle other correlations can be used. The first lag autocorrelation is represented typically by a complex number, in which the complex phase is proportional to the velocity of the sample. In the conventional method of scan conversion, the magnitude and phase of the first lag autocorrelation is interpolated between sampling points to generate pixel values for representing the flow velocity. Since these data are contained in reflected pulses, the magnitude of the measurable velocity is limited by the pulse rate frequency in accordance with the Nyquist sampling theorem. This limitation is the well-known "aliasing" phenomenon. In regions of large flow velocity, conventional methods can produce images where the flow velocity appears to suddenly reverse direction as a result of this aliasing problem. Furthermore, in regions where the flow velocity gradient is large, such as in the neighborhood of walls, the interpolating process can produce images with artificially irregular flow, again partly due to aliasing of the velocity data, as well as the inherent numerical inaccuracies of the interpolation itself.